Is it possible to do OpenCL rendering on Intel processors?
As it's still quite experimental and there are quite a few features still missing, it's only enabled for AMD gpus by default.  It's possible to &quot;force&quot; it to work on other opencl devices by setting the environment variable CYCLES_OPENCL_SPLIT_KERNEL_TEST=1
Linux
On Linux, run the following in a terminal:
CYCLES_OPENCL_SPLIT_KERNEL_TEST=1 blender

To make this permanent for all versions of blender which have the split kernel feature, put the following in your shell's initialization file (~/.profile or similar)
CYCLES_OPENCL_SPLIT_KERNEL_TEST=1; export CYCLES OPENCL_SPLIT_KERNEL_TEST

See this question for more detail.
Windows
On windows, create a batch file by putting the following in a text file:
set CYCLES_OPENCL_SPLIT_KERNEL_TEST=1
start blender
exit 

Rename the file so it has the .bat extension and put it in the same directory as the blender executable (if you installed blender with the installer, in C:\Program Files\Blender Foundation\Blender).
Then press  RMB on the .bat file and create a shortcut which you can put on your desktop or taskbar for ease of running it later.